The Best Time to Release Crayfish Fry and a Detailed Guide to Three Stocking Periods a Year

Crayfish is also known as red swamp crayfish, red claw crayfish and freshwater crayfish. When farming crayfish, fry are naturally released, so when is it relatively suitable to raise crayfish fry? What is the best time to release the fry?

Crayfish fry breeding

1. Broodstock selection

Broodstock can be selected in September-October of the previous year or March-April of the current year. They should weigh 30-50 g, have complete appendages, be healthy and disease-free, and be highly active, with a female-to-male ratio of 2-3:1. Male and female crayfish have obvious external features and are easy to distinguish.

(1) Among sexually mature crayfish, males are noticeably larger than females;

(2) Males have thick and large walking legs, with a bright red soft wart on the outer side of both ends of the legs; females have relatively smaller legs, and most female legs have no red soft wart, or even if present, the color is pale;

(3) The male genital pore opens at the base of the 5th pair of walking legs, while the female genital pore opens at the base of the 3rd pair of walking legs;

(4) Sexually mature females have an enlarged abdomen, while the abdomen of males is relatively narrow.

2. Broodstock cultivation

The broodstock pond should preferably have an area of about 1200 sq m, with loam soil as the bottom, a water depth of about 1 m, and a pond ridge width of more than 1.5 m, with sufficient and good water sources. Inlet and outlet openings should be built, and anti-escape walls should be built around the pond ridge with plastic film or calcium-plastic boards. 7-10 days before stocking, disinfect the pond with 50-60 kg/mu quicklime when the pond is dry. After disinfection, filter the water (to prevent wild fish from entering the pond) and fill to a depth of about 1 m, then apply 500-800 kg/mu decomposed livestock and poultry manure to fertilize the water. Place hidden objects in the pond for crayfish to climb and inhabit, such as branches, tree roots, and bamboo tubes, and transplant some aquatic plants.

Broodstock ponds can be stocked with 40-60 kg/mu of broodstock. Before stocking, soak them in 3% salt water for 10 minutes to kill pathogens. To fully utilize the water body and regulate water quality, silver carp and bighead carp can also be polycultured at 50-100 fish/mu. During the cultivation period, fresh aquatic plants, soybean cake, wheat bran or compound feed can be fed, along with some animal-based feed, such as chopped snail and mussel meat, and livestock and poultry slaughtering scraps. Daily feeding amount: 2%-3% of broodstock body weight in March, 4%-5% in April, and 6%-8% after May. Feed once each morning and evening, mainly in the evening, accounting for 70% of the daily ration. At the same time, strengthen water quality management, change new water once every 10-15 days, each time replacing 1/3; every 20 days, splash quicklime at 10-25 g/sq m mixed with water once to maintain good water quality and promote gonad development of broodstock.

3. Broodstock reproduction

In April-May, when the water temperature is above 20, broodstock begin mating. Fertilized eggs hatch into larvae in the female's abdomen, and hatching takes 40-70 days. After hatching, the larvae complete the growth and development process of the juvenile stage under the protection of the mother. Once the larvae leave the mother, they can actively feed and live independently. When a large number of larvae appear in the breeding pond, fry should be collected in time for fry cultivation.

4. Broodstock overwintering

The lowest survival water temperature for crayfish is 5℃. But when the water temperature is below 9℃, crayfish about 3 cm long have a very high mortality rate during overwintering; although adult crayfish can survive, mass death will also occur after 2-3 months. Therefore, doing a good job in broodstock overwintering and ensuring the water temperature remains at 16℃-18℃ during overwintering is also an important part of the entire breeding work.

The overwintering methods for broodstock are the same as those for Macrobrachium rosenbergii. Commonly used overwintering methods include covering the pond with plastic film for insulation, heating with electric heaters, overwintering with hot spring water, overwintering with factory waste heat water, and overwintering in glasshouses, all of which can achieve the effect of safe overwintering for broodstock.

5. Fry rearing techniques

Although crayfish do not carry a large number of eggs, the fertilization rate can be above 98%, and the hatching rate can reach 80%-85%. The problem is that after emergence, juvenile shrimp grow and molt frequently and are highly susceptible to environmental conditions, which affects the fry rearing rate. To improve the fry rearing rate, the key is to do the following well:

(1) Water quality requirements

During crayfish breeding, the water body should remain relatively stable, the water quality fresh, and the pH between 6.5-8; prevent excessive diurnal temperature differences; dissolved oxygen in the water should remain above 5.6 mg/L.

(2) Water body environment

There must be water hyacinth on the water surface (accounting for 1/3), and preferably aquatic plants at the bottom, as well as hidden burrows, to increase attachment sites for fry molting and to facilitate checking and grasping the emergence time and shrimp growth through washing fry with water hyacinth. Add grilles and filter nets at the water inlet to prevent harmful organisms from entering the pond, and also prevent frogs from entering the pond to lay eggs, avoiding tadpoles eating the fry. At the same time, prevent broodstock from climbing and escaping.

(3) Strengthen inspection

Persist in checking emergence in the morning and evening. When juvenile shrimp leave the mother, promptly capture the broodstock and return them to the broodstock pond for further cultivation, minimizing counting and transferring between ponds, and handle them with special care to avoid affecting egg-bearing broodstock and newly hatched juvenile shrimp.

(4) Ensure feed supply

Cultivate rotifers and other small zooplankton in a timely manner for the newly hatched juvenile shrimp to feed on. An estimated 3-5 days before emergence, begin catching a small amount of small zooplankton from a dedicated feed pond into the fry pond. Also use cooked egg yolk, soybean milk, etc. to promptly supplement the food supply needed by juvenile and young shrimp.

Larval shrimp cultivation

I. Cultivation requirements

Larval shrimp leave the mother to live, generally in a larval shrimp pond. After all larval shrimp have left the mother, remove the broodstock and leave the larval shrimp for cultivation. At this time, the average body length of the larval shrimp is 0.8 cm. After 15 days of cultivation, the body length can reach 2.4-3 cm, for farmers to carry out adult shrimp farming production.

(1) Pond requirements

Larval shrimp ponds are best as cement ponds with an area of 20-40 sq m and a water depth of 0.6-0.8 m. Earthen ponds are also acceptable, but newly built cement ponds must first undergo dealkalization treatment. They should be close to a water source, have good water quality, convenient inlet and drainage, and well-built anti-escape facilities, and undergo pond disinfection (the method is the same as for broodstock cultivation). After disinfection, apply 500 kg/mu fermented organic fertilizer to cultivate plankton for the larval shrimp to feed on. At the same time, set up tree roots, bamboo tubes, etc., to provide places for larval shrimp to inhabit, molt and hide.

(2) Water requirements

(1) The water quality is generally river water or well water, and the water should be fresh. At the water inlet, use a screen to filter the incoming water to prevent insects, aquatic animals (such as water centipedes), small fish and shrimp, and eggs from entering the pond. During the cultivation period, depending on the condition of waste and residual feed and water quality (such as high ammonia nitrogen and other three-state nitrogen), regularly siphon waste and change water to maintain good water quality.

(2) The suitable water temperature range is 27℃-29℃, and the variation range cannot exceed 2℃. If the water temperature is below 20℃, the growth rate slows down, which will seriously affect the survival rate. Therefore, the water temperature should remain relatively stable throughout the cultivation period.

(3) Dissolved oxygen: after larval shrimp leave the mother, the cultivation pond should be aerated continuously all day so that dissolved oxygen in the water is sufficient.

(4) pH: the pH of the cultivation water should be controlled at 7-8.5, slightly alkaline. If the water is found to be acidic, quicklime can be used to adjust the pH. For small-scale production breeding farms using tap water, baking soda can be used to adjust the pH.

2. Cultivation techniques

The stocking density of larval shrimp is generally 150-230 shrimp/sq m. Note that the fry specifications in the same pond should be consistent, and choose to release them on a sunny morning or a cloudy day. In the first week after stocking, soybean milk can be fed 3-4 times a day; from the second week onward, mainly feed animal-based feed such as small fish and shrimp, snail and mussel meat, earthworms, and silkworm pupae, appropriately mixed with corn, wheat, etc. made into a paste feed, fed once each morning and evening, with the morning feeding accounting for 40% of the daily ration. The early daily feeding amount is 0.25-0.40 kg per 10,000 larval shrimp, and afterward feed at about 10% of shrimp weight. During the cultivation period, change water once every 10 days, each time replacing 1/3; every 20 days splash quicklime water once at a concentration of about 20 g/cu m to regulate water quality. After 25-30 days of cultivation, the young shrimp body length can reach 3 cm, and they can be transferred to adult shrimp farming.

Three crayfish fry stocking periods a year

The first time to release crayfish fry:

From late March to early April, use overwintering fry from good-breed pond outlets, stocking 4000 crayfish fry per mu. Before releasing the fry, arrange the aquatic plants or bamboo leaf vegetables in the pond, accounting for about 50% of the total pond area. In early spring, alternately feed fresh chopped small miscellaneous fish and shrimp compound feed, feeding 1.5 kg dry feed per mu per day (3 kg fresh chopped fish pieces), and gradually increase the feed amount, increasing by 0.5 kg per mu every 5 days, until 1 month after fry release, feeding 5 kg per mu per day. 45 days after fry release, check with nets, and when the crayfish reach 34-36 shrimp/kg, they can be harvested.

The second time to release crayfish fry:

From late May to early June, after clearing the pond and removing wild species, the late spring fry are relatively large at this time, generally 120-160 shrimp/kg. Stock about 22.5 kg per mu, or 4000 crayfish fry. After fry release, feed boiled wheat grains (add 1% salt water and boil until the grains split open), feeding 5-7.5 kg per day, and stop feeding on rainy days. At the same time, plant bamboo leaf vegetables in early June, covering about 50% of the total water area. For the second crop of crayfish farming, attention should be paid to preventing escape and frequently changing and replenishing new water, and harvest and market before early September and before river crabs are listed. In the middle and late stages of the second crop of shrimp farming (starting 50 days after fry release), set cages daily to catch large ones and leave small ones, screening and harvesting crayfish for market.

The third time to release crayfish fry:

From mid-to-late September to early October, release early fry from autumn breeding, appropriately increase the stocking rate per mu, stocking about 5000 fry per mu of water surface, and choose medium and small specification fry of 200-300 shrimp/kg. Feed processed fresh miscellaneous fish pieces and shrimp feed to promote broodstock fattening before winter. The feeding amount depends on weather and water temperature. When the weather is good and the water temperature is above 20℃, feed 2.5-4 kg per mu per day (double the amount if fresh feed). When the water temperature is below 20℃, reduce the feeding amount by 50%; when the water temperature is below 15℃, reduce the feeding amount by another 50%; when the water temperature is below 10℃, stop feed feeding. When the average daily water temperature is below 10℃, increase the pond water injection to reach the highest water level of the pond, and retain water for overwintering. When the pond water temperature gradually rises again the next year (early March), harvest in a timely manner.